Polyolefins, such as polyethylene (PE), is synthesized by polymerizing olefin, such as ethylene (CH2═CH2), monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Olefin polymerizations are frequently carried out in a loop reactor using monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash vessel, where most of the diluent and unreacted monomers are flashed off and recycled.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be extruded and pelletized.
Polymerization of ethylene involves the polymerization of ethylene monomer in the reactor in the presence of a polymerization catalyst and optionally, if required depending on the used catalyst, an activating agent. Suitable catalysts for the preparation of polyethylene, comprise chromium-type catalysts, Ziegler-Matta catalysts and metallocene catalysts. Typically, the catalyst is used in particulate form.
Several systems have been disclosed which involve the preparation and the supply of catalyst slurry to a polymerization reaction. In general, for preparing catalyst slurry, a mixture of dry solid particulate catalyst and diluent are apportioned in a catalyst mixing vessel and thoroughly mixed. Then such catalyst slurry is typically transferred to a polymerization reactor for contact with the monomer reactants, generally under high pressure conditions.
It is known in the art that for the production of ethylene polymers with suitable properties it is important during polymerization to control reaction conditions, including reaction temperatures, reactant concentration, etc. Polymerization reactions are also sensitive to the quantity and the type of catalyst utilized.
Complications may occur during production of polyolefins, particularly polyethylenes. It is important to control reaction conditions, including reaction temperature and reactant concentrations, to obtain polyolefins with suitable properties. Polymerization reactions may also be sensitive to the quantity and the type of catalyst utilized. Underdosing or failure to supply catalyst lead to an insufficient and uneconomical polymerization process while overdosing of catalyst may lead to dangerous run-away reactions. Generally, incorrect dosing of catalyst may lead to suboptimal reaction conditions and/or to unexpected and sometimes extended down-time of the ethylene polymerization reactor.
Consequently, there remains a need in the art for ensuring that an adequate amount of catalyst is made available for production of polyolefin in order to reduce production costs, control process conditions and/or produce high-yielding, high-quality end-products.